I kind of think Gundam is cheating, as it keeps coming back in different forms. I am sure that there will always be a Gundam around, but it's more about the franchise than any particular series. It's hard to predict this kind of thing. There was a time when Bubblegum Crisis and Slayers seemed like they would be relevant forever. I'd say Cowboy Beebop and Ghost in the Shell SAC have legs.

Gundam
Sailor Moon...I mean it is getting a reboot sometime.
DBZ: all WSJ writers should be giving Toriyama a percentage every fucking week
Naruto: It won't die until Sasuke and Naruto go all brokeback cowboy. Kishi and Kubo just need to STAHP.
Bleach: See above, though it's nowadays most people talk about it as in 'Jesus Christ this needs to end'.
One Piece: This is the only shonen series that will probably be STILL going on ten years from now.

Anybody else think Serial Experiments Lain? Even though a lot of people don't talk about it anymore, i still feel like it's a classic.

I agree that it is a classic.

While it may be a more obscure title, the themes of increased presence on the Wired/Internet are echoed in our daily lives today. (Almost to a frightening extent). I think that is part of the reason why the series remains relevant 15 years later.